VI. Analysis of the Release Catalogs

j. XSC Photometric Spatial Uniformity

The Level-1 specification for photometric uniformity is
better than 10% over the sky. The 2MASS data appear
to meet this requirement, although further tests are needed.

Testing the uniformity is not straightforward, because
gradual drifts in, for example, calibration or airmass effects are difficult to
isolate. While external checks are useful, none are available which are
themselves immune from these problems, and using these introduces further
uncertainties, because they are typically limited to small regions
of the sky, and the measurements are carried out using different methods.

One internal check in 2MASS is galaxy colors. Because the galaxy
population exhibits a narrow range of (J-Ks) colors at a fixed
redshift, large samples of galaxies with the same magnitudes
have the same mean color. ([J-H] colors have very little dependence on
redshift or magnitude, so they do not provide a similar test.)
Extinction by interstellar dust reddens the colors, so we
limit the analysis here to sources at |b|>30°.
Two obvious areas for concern are differences between the north and the
south, and airmass effects at the celestial poles.

We selected a set of galaxies somewhat arbitrarily by requiring the
7´´-circular-aperture J magnitudes to be between 14 and 15.
(This range was chosen to give small errors of the mean and large
magnitude-dependent color changes below.)
Comparing the mean (J-Ks) colors for the northern and southern
galaxies, no significant differences were apparent:

mean value

north-south

error of the mean

(J-Ks) in 7´´ aperture

1.1192

0.0006

±0.0032

The errors of the mean listed here are based just on the uncertainties
in calculating the mean values for north and south, given the rms variations
of the values.

The significance of this is that the (J-Ks) color differences are
very small, compared to the K-correction (redshift effect): the mean color for
galaxies 0.1 mag fainter or brighter (i.e., between 13.9 and 14.9, or between
14.1 and 15.1) shift by 0.0072 mag redder or bluer. Thus, the lack of
(J-Ks) color difference implies that
the southern and northern galaxies truly represent objects from the same
magnitude range, to within 0.044 mag [=(0.0032/0.0072)×0.1 mag].

We can also test for color effects near the celestial poles. Here
we are limited in the current accuracy of the test, because of insufficient
number statistics, and the poles are themselves at fairly low Galactic
latitude. Nevertheless, comparing galaxies within 20° of the pole with those
that pass within 20° of the zenith (and requiring all to be
between Galactic latitudes of 20° and 35°), we find the following
colors:

mean value

zenith-pole

error of the mean

(J-Ks) in 7´´ aperture

1.1310

-0.0014

± 0.0078

(J-H) in 7´´ aperture

0.7017

-0.0001

± 0.0073

This comparison shows no significant difference in zenith and pole color
measurements. Note, though, that the mean J-Ks colors do show
evidence of reddening, relative to the high Galactic latitude measurements
earlier. The J-H colors here are particularly useful for showing the lack of a
color effect, since the mean (J-H) color for galaxies is nearly constant as a
function of magnitude.